Opsin synthesis was measured in Irish setters affected with a recessively inherited retinal dysplasia. Photoreceptors fail to differentiate normally and cyclic GMP levels are greatly elevated during differentiation prior to the time when the photoreceptors degenerate. However, opsin synthesis was found to occur at a normal rate at a time when cyclic GMP levels had already risen substantially but morphological changes were not yet evident. Thus, no generalized abnormality in protein synthesis was detected which suggests that the elevation in cyclic GMP, caused by a defect in the synthesis of a specific phosphodiesterase, may represent the primary biochemical abnormality.